Should
your morning cup of coffee be added to the arsenal of weapons to stop slugs and
snails? USDA-ARS scientists reported in the June 27, 2002 issue of the
scientific journal Nature that caffeine can repel or kill snails. An environmentally acceptable, natural compound, caffeine has great potential
as an alternative to today's snail- and slug-killing chemicals.
Researchers
of ornamental plant
pests in Hawaii were testing various compounds against the introduced coqui
frog that infests potted
plants, when they noticed that caffeine solutions did a good job of killing the little
orchid snail, Zonitoides arboreus. This common snail is fairly hard to control with conventional pesticides
that kill molluscs, including slugs and snails.
In preliminary experiments, conducted in a research greenhouse in Hilo, two-striped slugs (Veronicella cubensis) were allowed to bury themselves in soil in pots, then the soil was wetted with a 2% solution of caffeine in water. After 3½ hours only 25% of the slugs remained in the soil, and by 2 days all of the slugs were gone, with 92% dead.
When slugs were fed 2% caffeine-treated cabbage leaves, 39% less was consumed over a 4 day period. And when slugs had a choice of treated or untreated leaves, consumption of the treated leaves was reduced by 77% (1% treatment) or 64% (2% treatment). Total cabbage consumption was reduced by 14 and 28%, respectively, indicating caffeine significantly reduces feeding.
Then
greenhouse studies were conducted in Hilo, HA, applying caffeine solution sprays to
snail-infested coir (coconut husk-chip medium in which potted orchids are
grown). One and 2% solutions killed 60 and 95%, respectively, of the snails in
the coir. And the 2% solution was even more effective in reducing the presence
of snails than the commercial
standard for orchid-snail treatment. After 30 days, caffeine-treated coir had
only 5 snails, while coir treated with a standard dose of commercial metaldehyde
had 35 snails and the water control 43 snails.
The researchers do not know how caffeine kills slugs and snails. But tests showed topically applied 2% caffeine solutions reduced orchid snail heart contractions, and all died after 4 days. Caffeine's high solubility in water — a key component of molluscs' locomotor mucus — could make slugs and snails more susceptible to contact poisoning from caffeine than other animals such as arthropods.
But caffeine may not be the perfect control for slugs on all plants. Although the researchers found that 2% caffeine does not damage the foliage of Dracaena, Anthurium, palms or orchids, it did cause leaf yellowing on ferns, bromeliads and lettuce.
The strength of caffeine that repelled the slugs was about the same as that in strong coffee. (The researchers used caffeine powder, which is sold as a pure product for adding to cola-type drinks.) But before you start brewing some java to pour around your hostas, remember that these experiments were not done with slugs or plants in the Midwest, which could produce different results than those seen in Hawaii on tropical plants. Also, before caffeine can legally be used as a pesticide, it must go through the Environmental Protection Agency registration process. But maybe some enterprising company will try to commercialize this!
–Susan Mahr, University of Wisconsin
